Method and apparatus for transmitting indication of cell coverage in wireless communication system

ABSTRACT

A method and apparatus for transmitting an indication of cell coverage in a wireless communication system is provided. A first eNodeB (eNB) transmits an indication which indicates cell coverage of an active antenna system (AAS) to a second eNB. The indication may be one of a cell split/merge/remove indication.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT International ApplicationNo. PCT/KR2014/006266, filed on Jul. 11, 2014, which claims priorityunder 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/845,377,filed on Jul. 12, 2013 and U.S. Provisional Application No. 61/883,203,filed on Sep. 27, 2013, all of which are hereby expressly incorporatedby reference into the present application.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for transmitting an indicationof cell coverage in a wireless communication system.

BACKGROUND ART

Universal mobile telecommunications system (UMTS) is a 3rd generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). The long-term evolution (LTE) of UMTS is under discussion by the3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3GPP LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighboring eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical EPC. Referring to FIG. 2, the eNB 20 may perform functions ofselection for gateway 30, routing toward the gateway 30 during a radioresource control (RRC) activation, scheduling and transmitting of pagingmessages, scheduling and transmitting of broadcast channel (BCH)information, dynamic allocation of resources to the UEs 10 in both ULand DL, configuration and provisioning of eNB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE_IDLE state management,ciphering of the user plane, SAE bearer control, and ciphering andintegrity protection of NAS signaling.

FIG. 3 shows a block diagram of a user plane protocol stack and acontrol plane protocol stack of an LTE system. FIG. 3-(a) shows a blockdiagram of a user plane protocol stack of an LTE system, and FIG. 3-(b)shows a block diagram of a control plane protocol stack of an LTEsystem.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel. Data istransferred between the MAC layer and the PHY layer through thetransport channel. Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

FIG. 4 shows an example of a physical channel structure. A physicalchannel consists of a plurality of subframes in time domain and aplurality of subcarriers in frequency domain. One subframe consists of aplurality of symbols in the time domain. One subframe consists of aplurality of resource blocks (RBs). One RB consists of a plurality ofsymbols and a plurality of subcarriers. In addition, each subframe mayuse specific subcarriers of specific symbols of a corresponding subframefor a PDCCH. For example, a first symbol of the subframe may be used forthe PDCCH. The PDCCH carries dynamic allocated resources, such as aphysical resource block (PRB) and modulation and coding scheme (MCS). Atransmission time interval (TTI) which is a unit time for datatransmission may be equal to a length of one subframe. The length of onesubframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel. A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving an RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that can be mapped to the UL-SCH, the DTCH that can bemapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that can be mapped to the BCH or DL-SCH, the PCCH thatcan be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, andthe DTCH that can be mapped to the DL-SCH, the MCCH that can be mappedto the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom a higher layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, can be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

Referring to FIG. 3-(a), the RLC and MAC layers (terminated in the eNBon the network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARM). ThePDCP layer (terminated in the eNB on the network side) may perform theuser plane functions such as header compression, integrity protection,and ciphering.

Referring to FIG. 3-(b), the RLC and MAC layers (terminated in the eNBon the network side) may perform the same functions for the controlplane. The RRC layer (terminated in the eNB on the network side) mayperform functions such as broadcasting, paging, RRC connectionmanagement, RB control, mobility functions, and UE measurement reportingand controlling. The NAS control protocol (terminated in the MME ofgateway on the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

An RRC state indicates whether an RRC layer of the UE is logicallyconnected to an RRC layer of the E-UTRAN. The RRC state may be dividedinto two different states such as an RRC connected state and an RRC idlestate. When an RRC connection is established between the RRC layer ofthe UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, andotherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has theRRC connection established with the E-UTRAN, the E-UTRAN may recognizethe existence of the UE in RRC_CONNECTED and may effectively control theUE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN,and a CN manages the UE in unit of a TA which is a larger area than acell. That is, only the existence of the UE in RRC_IDLE is recognized inunit of a large area, and the UE must transition to RRC_CONNECTED toreceive a typical mobile communication service such as voice or datacommunication.

In RRC_IDLE state, the UE may receive broadcasts of system informationand paging information while the UE specifies a discontinuous reception(DRX) configured by NAS, and the UE has been allocated an identification(ID) which uniquely identifies the UE in a tracking area and may performpublic land mobile network (PLMN) selection and cell re-selection. Also,in RRC_IDLE state, no RRC context is stored in the eNB.

In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and acontext in the E-UTRAN, such that transmitting and/or receiving datato/from the eNB becomes possible. Also, the UE can report channelquality information and feedback information to the eNB. InRRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs.Therefore, the network can transmit and/or receive data to/from UE, thenetwork can control mobility (handover and inter-radio accesstechnologies (RAT) cell change order to GSM EDGE radio access network(GERAN) with network assisted cell change (NACC)) of the UE, and thenetwork can perform cell measurements for a neighboring cell.

In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically,the UE monitors a paging signal at a specific paging occasion of everyUE specific paging DRX cycle. The paging occasion is a time intervalduring which a paging signal is transmitted. The UE has its own pagingoccasion.

A paging message is transmitted over all cells belonging to the sametracking area. If the UE moves from one TA to another TA, the UE willsend a tracking area update (TAU) message to the network to update itslocation.

When the user initially powers on the UE, the UE first searches for aproper cell and then remains in RRC_IDLE in the cell. When there is aneed to establish an RRC connection, the UE which remains in RRC_IDLEestablishes the RRC connection with the RRC of the E-UTRAN through anRRC connection procedure and then may transition to RRC_CONNECTED. TheUE which remains in RRC_IDLE may need to establish the RRC connectionwith the E-UTRAN when uplink data transmission is necessary due to auser's call attempt or the like or when there is a need to transmit aresponse message upon receiving a paging message from the E-UTRAN.

It is known that different cause values may be mapped o the signaturesequence used to transmit messages between a UE and eNB and that eitherchannel quality indicator (CQI) or path loss and cause or message sizeare candidates for inclusion in the initial preamble.

When a UE wishes to access the network and determines a message to betransmitted, the message may be linked to a purpose and a cause valuemay be determined. The size of the ideal message may be also bedetermined by identifying all optional information and differentalternative sizes, such as by removing optional information, or analternative scheduling request message may be used.

The UE acquires necessary information for the transmission of thepreamble, UL interference, pilot transmit power and requiredsignal-to-noise ratio (SNR) for the preamble detection at the receiveror combinations thereof. This information must allow the calculation ofthe initial transmit power of the preamble. It is beneficial to transmitthe UL message in the vicinity of the preamble from a frequency point ofview in order to ensure that the same channel is used for thetransmission of the message.

The UE should take into account the UL interference and the UL path lossin order to ensure that the network receives the preamble with a minimumSNR. The UL interference can be determined only in the eNB, andtherefore, must be broadcast by the eNB and received by the UE prior tothe transmission of the preamble. The UL path loss can be considered tobe similar to the DL path loss and can be estimated by the UE from thereceived RX signal strength when the transmit power of some pilotsequence of the cell is known to the UE.

The required UL SNR for the detection of the preamble would typicallydepend on the eNB configuration, such as a number of Rx antennas andreceiver performance. There may be advantages to transmit the ratherstatic transmit power of the pilot and the necessary UL SNR separatelyfrom the varying UL interference and possibly the power offset requiredbetween the preamble and the message.

The initial transmission power of the preamble can be roughly calculatedaccording to the following formula:Transmit power=TransmitPilot−RxPilot+ULInterference+Offset+SNRRequired

Therefore, any combination of SNRRequired, ULInterference, TransmitPilotand Offset can be broadcast. In principle, only one value must bebroadcast. This is essentially in current UMTS systems, although the ULinterference in 3GPP LTE will mainly be neighboring cell interferencethat is probably more constant than in UMTS system.

The UE determines the initial UL transit power for the transmission ofthe preamble as explained above. The receiver in the eNB is able toestimate the absolute received power as well as the relative receivedpower compared to the interference in the cell. The eNB will consider apreamble detected if the received signal power compared to theinterference is above an eNB known threshold.

The UE performs power ramping in order to ensure that a UE can bedetected even if the initially estimated transmission power of thepreamble is not adequate. Another preamble will most likely betransmitted if no ACK or NACK is received by the UE before the nextrandom access attempt. The transmit power of the preamble can beincreased, and/or the preamble can be transmitted on a different ULfrequency in order to increase the probability of detection. Therefore,the actual transmit power of the preamble that will be detected does notnecessarily correspond to the initial transmit power of the preamble asinitially calculated by the UE.

The UE must determine the possible UL transport format. The transportformat, which may include MCS and a number of resource blocks thatshould be used by the UE, depends mainly on two parameters, specificallythe SNR at the eNB and the required size of the message to betransmitted.

In practice, a maximum UE message size, or payload, and a requiredminimum SNR correspond to each transport format. In UMTS, the UEdetermines before the transmission of the preamble whether a transportformat can be chosen for the transmission according to the estimatedinitial preamble transmit power, the required offset between preambleand the transport block, the maximum allowed or available UE transmitpower, a fixed offset and additional margin. The preamble in UMTS neednot contain any information regarding the transport format selected bythe EU since the network does not need to reserve time and frequencyresources and, therefore, the transport format is indicated togetherwith the transmitted message.

The eNB must be aware of the size of the message that the UE intends totransmit and the SNR achievable by the UE in order to select the correcttransport format upon reception of the preamble and then reserve thenecessary time and frequency resources. Therefore, the eNB cannotestimate the SNR achievable by the EU according to the received preamblebecause the UE transmit power compared to the maximum allowed orpossible UE transmit power is not known to the eNB, given that the UEwill most likely consider the measured path loss in the DL or someequivalent measure for the determination of the initial preambletransmission power.

The eNB could calculate a difference between the path loss estimated inthe DL compared and the path loss of the UL. However, this calculationis not possible if power ramping is used and the UE transmit power forthe preamble does not correspond to the initially calculated UE transmitpower. Furthermore, the precision of the actual UE transmit power andthe transmit power at which the UE is intended to transmit is very low.Therefore, it has been proposed to code the path loss or CQI estimationof the downlink and the message size or the cause value in the UL in thesignature.

Self-organizing networks (SON) enhancements are necessary for theinteroperability of the existing features as well as for the newfeatures and new deployments considered in 3GP LTE rel-12. In 3GPP LTErel-11, mobility robustness optimization (MRO) has been enhanced toidentify for which UE type the failure has occurred. Other SON use casesmight require similar enhancements. For example, mobility load balancing(MLB) is not able to distinguish between UEs that support cell rangeexpansion (CRE) and non-CRE UEs.

Active antennas allow the creation of multiple vertical and horizontalbeams making the deployment dynamic. That enables dynamic cellsplitting/merging to handle changing load conditions. For example, beamsmay be steered to distribute capacity precisely according to actualtraffic mix, traffic location and user demands. That makes activeantennas particularly good for suburban and rural areas, where fixeddeployment of pico cells is expensive, but the network may facecongestion situations nonetheless. SON can automate the networkdeployment based on active antennas.

SON enhancements and new features for the deployments based on activeantenna system (AAS) have been discussed. Possible deployment scenariosof an AAS, and additionally required SON features for the AAS needs tobe discussed.

SUMMARY OF INVENTION Technical Problem

The present invention provides a method and apparatus for transmittingan indication of cell coverage in a wireless communication system. Thepresent invention provides a method for notifying neighbor eNodeBs(eNBs) of a plan to split or merge a cell in an active antenna system(AAS).

Solution to Problem

In an aspect, a method for transmitting, by a first eNodeB (eNB), anindication of cell coverage in a wireless communication system isprovided. The method includes transmitting an indication which indicatescell coverage of an active antenna system (AAS) to a second eNB.

The indication may be a cell split indication which indicates that acell, managed by the first eNB, has been split. In this case, the methodmay further include splitting the cell before transmitting theindication to the second eNB.

The indication may be a cell split indication which indicates that acell, managed by the first eNB, is to be split. In this case, the methodmay further include splitting the cell after transmitting the indicationto the second eNB.

The indication may be a cell merge indication which indicates thatcells, managed by the first eNB, have been merged to one cell. In thiscase, the method may further include merging the cells to one cellbefore transmitting the indication to the second eNB.

The indication may be a cell merge indication which indicates thatcells, managed by the first eNB, are to be merged. In this case, themethod may further include merging the cells to one cell aftertransmitting the indication to the second eNB.

The indication may be a cell remove indication which indicates that acell, managed by the first eNB, has been removed or is to be removed.

In another aspect, a first eNodeB (eNB) in a wireless communicationsystem is provided. The first eNB includes a radio frequency (RF) unitfor transmitting or receiving a radio signal, and a processor coupled tothe RF unit, and configured to transmit an indication which indicatescell coverage of an active antenna system (AAS) to a second eNB.

In another aspect, a method for transmitting, by a first eNodeB (eNB),an indication of cell coverage in a wireless communication system isprovided. The method includes determining cell coverage of an activeantenna system (AAS), receiving a handover request message from a secondeNB right after determining cell coverage of the AAS, and transmitting ahandover preparation failure message including cause information, whichindicates cell coverage of the AAS, to the second eNB.

Advantageous Effects of Invention

AAS operations can be supported efficiently.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical EPC.

FIG. 3 shows a block diagram of a user plane protocol stack and acontrol plane protocol stack of an LTE system.

FIG. 4 shows an example of a physical channel structure.

FIG. 5 shows options for AAS-based deployment.

FIG. 6 shows an example of cell split operations among eNBs.

FIG. 7 shows an example of a method or transmitting an indication ofcell coverage according to an embodiment of the present invention.

FIG. 8 shows an example of a method for transmitting a cell splitindication according to an embodiment of the present invention.

FIG. 9 shows an example of a method for transmitting a cell mergeindication according to an embodiment of the present invention.

FIG. 10 shows an example of a method for transmitting a cell removeindication according to an embodiment of the present invention.

FIG. 11 shows another example of a method for transmitting a cell splitindication according to an embodiment of the present invention.

FIG. 12 shows another example of a method for transmitting a cell mergeindication according to an embodiment of the present invention.

FIG. 13 shows another example of a method for transmitting a cell removeindication according to an embodiment of the present invention.

FIG. 14 shows a wireless communication system to implement an embodimentof the present invention.

MODE FOR THE INVENTION

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is an evolution of IEEE 802.16e, and provides backwardcompatibility with an IEEE 802.16-based system. The UTRA is a part of auniversal mobile telecommunication system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is a part of anevolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA indownlink and uses the SC-FDMA in uplink LTE-advance (LTE-A) is anevolution of the 3GPP LTE.

For clarity, the following description will focus on the LTE-A. However,technical features of the present invention are not limited thereto.

An active antenna system (AAS) refers to a base station (BS) equippedwith an antenna array system, the radiation pattern of which may bedynamically adjustable. The AAS introduces an alternative antenna systemfrom the one installed in the conventional BS. The interactions betweenthe antenna array system and the transmitters and receivers within theAAS might be different from the conventional BS and the conventionalantenna system.

AAS deployment scenarios are described. It may be referred to Section 5of 3GPP TR 37.840 V12.0.0 (2013-03).

1) Tilt and Radiation Pattern Control

Antennas are usually manufactured with a fixed beamwidth, and antennamanufacturers typically offer a limited number of beamwidth variationswithin their conventional product lines. Conventional BS installationsoften introduce physical tilt to the antenna in order to orient the mainlobe of the antenna response towards the ground. Antenna tilt isselected to optimize desired cell coverage and to minimize interferenceto and from adjacent cells. Some installations employ remote electricaltilt (RET) devices which allow adjustment of the phase shift tofacilitate remote control of the antenna tilt angle.

An AAS may dynamically control the elevation and azimuth angles, as wellas the beamwidth of its radiation pattern via electronic means.Electronic control may be used along with mechanical control. The AASradiation pattern may be adapted to the specific deployment scenario andpossibly to changing traffic patterns. The AAS radiation pattern mayalso be independently optimized for different links such asindependently for uplink and downlink, for coverage and beam forminggain purposes.

The concepts of tilt and beamwidth control can be extended by atechnique known as cell partitioning in which the cell is subdivided invertical or horizontal directions by adjustment of the antenna pattern.For example, one cell partition is located close to the BS and the othercell partition is located farther away from the BS.

2) Multiple-Input Multiple-Output (MIMO)

MIMO is a general terminology that includes the various spatialprocessing techniques such as beamforming, diversity, and spatialmultiplexing. Brief description of each is provided below.

-   -   Beamforming: The use of a dedicated beam formed towards the UE        when data demodulation using a dedicated reference signal is        supported by the UE.    -   Diversity: The use of diversity techniques to jointly optimize        in the spatial and frequency domain through the use of, for        example, spatial-frequency block code (SFBC) or frequency        switching transmit diversity (FSTD), or combinations of both.    -   Spatial multiplexing: The transmission of multiple signal        streams to one (SU-MIMO) or more (MU-MIMO) UEs using multiple        spatial layers created by combinations of the available        antennas.

3) Differentiated Antenna Behaviors at Different Carrier Frequencies

AAS supports the use of different antennas at different carrierfrequencies and for different radio access technologies (RATs). Forexample, an AAS may create 4 virtual antennas for an LTE carrier and 2antennas for a GSM or high-speed packet access (HSPA) carrier.

4) Per RB (or UE) Transmission and Reception

Each UE may get its own beam that tracks the movement of the UE. Thecurrent specification support for spatial multiplexing, beamforming andtransmit diversity includes the ability to schedule transmission andreception to one UE within one resource block. This allows beamformingto individual UEs with adaptation to mobility, as an example.

The AAS BS can be deployed for wide area, medium range, and local areacoverage.

-   -   The wide area coverage deployment scenario is typically found in        outdoor macro environments, where the BS antennas are located on        masts, roof tops or high above street level. An AAS BS designed        for wide area coverage is called a macro AAS.    -   The medium range coverage deployment scenario is typically found        in outdoor micro environments, where the AAS BSs are located        below roof tops. An AAS BS designed for medium range coverage is        called a micro AAS.    -   The local area BS deployment scenario is typically found indoors        (offices, subway stations, etc.) where antennas are located on        ceilings or walls. Deployment scenarios for local area coverage        can also be found outdoors in hot spot areas like marketplaces,        high streets or railway stations. An AAS BS designed for local        area coverage is called a pico AAS.

The radiation pattern for an AAS BS can be dynamically adjustable, whilea fixed beam pattern is assumed for the conventional BS. Coexistence ofan AAS BS with a conventional BS based on an un-coordinated deploymentshall be considered. Analytical approaches can be used to study thecoexistence requirements based on existing results, supplemented withadditional simulations when necessary. The following initial scenariosare identified for the purpose of studying the spatial characteristicsfor AAS BS:

-   -   E-UTRA macro AAS BS co-located with another E-UTRA macro AAS BS    -   E-UTRA macro AAS BS co-located with E-UTRA macro legacy BS

Based on the deployment/coexistence scenarios described above, thedeployments scenarios of AAS may be categorized on three aspects asdescribed in Table 1 below.

TABLE 1 Amount of development Possible scenarios required further Cellpartitioning split to two parts (inner medium area and outer area) trackthe movement high medium of each UE Frequency and identical frequencyfor RAT assignment all partition in a cell different frequency formedium medium each partition different RAT for high each partitionCoexistence macro AAS BS co-located scenarios with another macro AAS BSmacro AAS BS co- medium located with macro legacy BS

Referring to Table 1, related to the cell partitioning, splitting a cellto inner part and outer part is considered as a basic deploymentscenario. If more accurate and adaptive beam steering is assumed, per UEtransmission and reception by tracking each UE's movement would bepossible scenario.

Related to frequency and RAT assignment for each cell partition, thereare three possible options: 1) assigning an identical frequency for allpartition, 2) different frequency for each partition, and 3) differentRAT for each partition. Among three options, assigning different RAT foreach partition may require more efforts in the network point of viewthan other options, e.g., inter-RAT handover, cell identifier (ID)allocation, and need more discussion about use cases.

Related to the coexistence scenarios, if the interworking SON mechanismsbetween AAS BSs co-located with each other are developed, then it can beused for the case of AAS BS co-located with legacy BS. However in thiscase the compatibility problems should be considered.

Therefore, it is preferable to focus on the following AAS deploymentscenarios first.

-   -   splitting cell to inner area and outer area,    -   assigning identical frequency for all partition in a cell or        different frequency for each partition, and    -   macro AAS BS co-located with another macro AAS BS.

Regarding AAS-based deployment, there are three possible options asfollows based on whether a cell is split or not, and if a cell is split,then whether a new physical cell identity (PCI) is assigned to the splitcell or not.

-   -   Option 1: Cell shaping/beamforming without splitting cell    -   Option 2: Cell splitting without PCI allocation    -   Option 3: Cell splitting with PCI allocation

FIG. 5 shows options for AAS-based deployment.

FIG. 5-(a) shows a case of option 1 described above. In option 1, theeNB uses beamforming to cover additional area. It has an effect toextend the cell coverage. Option 1 is not helpful to release the trafficload in the existing cell area.

FIG. 5-(b) shows a case of option 2 described above. Option 2 makes theeNB reuse resources at each split cell. Accordingly, the traffic loadcan be reduced by half ideally. However, option 2 has the problem thatthe UE at the border of two split cells cannot distinguish the signalfrom one split cell and the signal from the other split cell.

FIG. 5-(c) shows a case of option 3 described above. Option 3 assigns anindependent PCI to the split cell. Even though it increases the systemoperational complexity, option 3 has an advantage of reducing the celltraffic load without any confusion on UE side. Accordingly, ifconsidering the aspects of high UE density environment and UE servicestability, option 3 would be the most reasonable deployment scenario.However, the present invention described below according to embodimentsof the present invention does not limited to option 3. That is, all ofoption 1, 2, and 3 may be considered for the present invention.

FIG. 6 shows an example of cell split operations among eNBs. FIG. 6shows cell split operations among two eNBs, i.e., eNB1 and eNB2.Referring to FIG. 6, cell 1 managed by eNB1 is split into cell 1 a andcell 1 b. By splitting cell, the service capacity of eNB can beimproved. Thus, if the amount of the required traffic (or, the number ofUEs) in cell 1 of eNB1 is high, eNB1 can escape from the heavy loadstatus by splitting cell 1 into cell 1 a and cell 1 b. Moreover, if cell2 is in the heavy load status, it is possible that eNB1 splits cell 1and accepts UEs of cell 2.

The problem is that the practical cell coverage before splitting cellwould be different with the coverage area of split cells. Therefore, ifan eNB plans to split or merge its cells, it should notify neighbor eNBsof its plan. Alternatively, after cell split/merge operation iscompleted, the eNB should notify neighbor eNBs that its cells have beensplit or merged. In addition, when an eNB removes its cell (i.e., stopserving its cell), the eNB should notify its plan or the cell-removingevent to its neighbor eNBs as well.

FIG. 7 shows an example of a method or transmitting an indication ofcell coverage according to an embodiment of the present invention. Instep S100, the eNB1 transmits an indication which indicates cellcoverage of an AAS. The indication which indicates cell coverage of anAAS may be a cell split indication. The cell split indication may informa neighbor eNB the event that the eNB1 completed to split its cell, orthe plan to split its cell after making cell-splitting decision.Alternatively, the indication which indicates cell coverage of an AASmay be a cell merge indication. The cell merge indication may inform aneighbor eNB the event that the eNB1 completed to merge its cells to onecell, or the plan to merge its cells to one cell after makingcell-merging decision. Alternatively, the indication which indicatescell coverage of an AAS may be a cell remove indication. The cell removeindication may inform a neighbor eNB the event that the eNB1 completedto remove its cell, or the plan to remove its cell after makingcell-removing decision.

Hereinafter, various embodiments of the present invention are described.

FIG. 8 shows an example of a method for transmitting a cell splitindication according to an embodiment of the present invention.

Referring to FIG. 8-(a), in step S200, the eNB1 decides to split itscell, and split its cell. In step S201, the eNB1 transmits a cell spiltindication which indicates that the eNB1 has split its cell.Accordingly, the eNB1 may inform the eNB2 the event that the eNB1completed to split its cell.

Referring to FIG. 8-(b), in step S210, the eNB1 decides to split itscell. In step S211, the eNB1 transmits a cell spilt indication whichindicates that the eNB1 will split its cell. Accordingly, the eNB1 mayinform the eNB2 the plan to split its cell after making cell-splittingdecision. In step S212, the eNB1 splits its cell.

Information related to cell split operation may be acknowledged by theeNB1 and eNB2 in advance, or may be transmitted via the cell splitindication.

FIG. 9 shows an example of a method for transmitting a cell mergeindication according to an embodiment of the present invention.

Referring to FIG. 9-(a), in step S300, the eNB1 decides to merge splitcells, and merges the split cells. In step S301, the eNB1 transmits acell merge indication which indicates that the eNB1 has merged its splitcells. Accordingly, the eNB1 may inform the eNB2 the event that the eNB1completed to merge its split cells to one cell.

Referring to FIG. 9-(b), in step S310, the eNB1 decides to merge splitcells. In step S311, the eNB1 transmits a cell merge indication whichindicates that the eNB1 will merge its split cells. Accordingly, theeNB1 may inform the eNB2 the plan to merge its split cells to one cellafter making cell-merging decision. In step S312, the eNB1 merges itssplit cells.

Information related to cell merge operation may be acknowledged by theeNB1 and eNB2 in advance, or may be transmitted via the cell mergeindication.

FIG. 10 shows an example of a method for transmitting a cell removeindication according to an embodiment of the present invention.

Referring to FIG. 10-(a), in step S400, the eNB1 decides to remove splitcells, and removes the split cells. In step S401, the eNB1 transmits acell remove indication which indicates that the eNB1 has removed itssplit cells. Accordingly, the eNB1 may inform the eNB2 the event thatthe eNB1 completed to remove its cell.

Referring to FIG. 10-(b), in step S410, the eNB1 decides to remove splitcells. In step S411, the eNB1 transmits a cell remove indication whichindicates that the eNB1 will remove its split cells. Accordingly, theeNB1 may inform the eNB2 the plan to remove its cell after makingcell-removing decision. In step S412, the eNB1 remove its cell.

Information related to cell remove operation may be acknowledged by theeNB1 and eNB2 in advance, or may be transmitted via the cell removeindication.

Moreover, if, right after an eNB decides to split, merge or remove itscell, the eNB may receive the HANDOVER REQUEST message from its neighboreNB. Then, the eNB needs to reject the handover request in order toprevent a handover failure, and inform the appropriate rejection causeto its neighbor eNB.

FIG. 11 shows another example of a method for transmitting a cell splitindication according to an embodiment of the present invention.

Referring to FIG. 11-(a), in step S500, the eNB1 decides to split itscell. In step S501, the eNB2 decides a handover, and accordingly, instep S502, the eNB2 transmits the HANDOVER REQUEST message to the eNB1.That is, the eNB1 receives the HANDOVER REQUEST message from the eNB2right after the eNB1 decided to split its cell. Therefore, in step S503,the eNB1 transmits the HANDOVER PREPARATION FAILURE message to the eNB2with the cause information to indicate that the target cell (e.g., cellof the eNB1) is planned to be split. In step S504, the eNB1 splits itscell. In step S505, the eNB1 transmits a cell spilt indication whichindicates that the eNB1 has split its cell. Accordingly, the eNB1 mayinform the eNB2 the event that the eNB1 completed to split its cell.

Referring to FIG. 11-(b), in step S510, the eNB1 decides to split itscell. In step S511, the eNB2 decides a handover, and accordingly, instep S512, the eNB2 transmits the HANDOVER REQUEST message to the eNB1.That is, the eNB1 receives the HANDOVER REQUEST message from the eNB2right after the eNB1 decided to split its cell. Therefore, in step S513,the eNB1 transmits the HANDOVER PREPARATION FAILURE message to the eNB2with the cause information to indicate that the target cell (e.g., cellof the eNB 1) is planned to be split. In step S514, the eNB1 transmits acell spilt indication which indicates that the eNB1 will split its cell.Accordingly, the eNB1 may inform the eNB2 the plan to split its cellafter making cell-splitting decision. In step S515, the eNB1 splits itscell.

FIG. 12 shows another example of a method for transmitting a cell mergeindication according to an embodiment of the present invention.

Referring to FIG. 12-(a), in step S600, the eNB1 decides to merge itssplit cells. In step S601, the eNB2 decides a handover, and accordingly,in step S602, the eNB2 transmits the HANDOVER REQUEST message to theeNB1. That is, the eNB1 receives the HANDOVER REQUEST message from theeNB2 right after the eNB1 decided to merge its split cells to one cell.Therefore, in step S603, the eNB1 transmits the HANDOVER PREPARATIONFAILURE message to the eNB2 with the cause information to indicate thatthe target cell (e.g., cell of the eNB1) is planned to be merged withother cell. In step S604, the eNB1 merges the split cells. In step S605,the eNB1 transmits a cell merge indication which indicates that the eNB1has merged its split cells. Accordingly, the eNB1 may inform the eNB2the event that the eNB1 completed to merge its split cells to one cell.

Referring to FIG. 12-(b), in step S610, the eNB1 decides to merge itssplit cells. In step S611, the eNB2 decides a handover, and accordingly,in step S612, the eNB2 transmits the HANDOVER REQUEST message to theeNB1. That is, the eNB1 receives the HANDOVER REQUEST message from theeNB2 right after the eNB1 decided to merge its split cells to one cell.Therefore, in step S613, the eNB1 transmits the HANDOVER PREPARATIONFAILURE message to the eNB2 with the cause information to indicate thatthe target cell (e.g., cell of the eNB1) is planned to be merged withother cell. In step S614, the eNB1 transmits a cell merge indicationwhich indicates that the eNB1 will merge its split cells. Accordingly,the eNB1 may inform the eNB2 the plan to merge its split cells to onecell after making cell-merging decision. In step S615, the eNB1 mergesits split cells.

FIG. 13 shows another example of a method for transmitting a cell removeindication according to an embodiment of the present invention.

Referring to FIG. 13-(a), in step S700, the eNB1 decides to remove itscell. In step S701, the eNB2 decides a handover, and accordingly, instep S702, the eNB2 transmits the HANDOVER REQUEST message to the eNB1.That is, the eNB1 receives the HANDOVER REQUEST message from the eNB2right after the eNB1 decided to remove its cell. Therefore, in stepS703, the eNB1 transmits the HANDOVER PREPARATION FAILURE message to theeNB2 with the cause information to indicate that the target cell (e.g.,cell of the eNB1) is planned to be removed. In step S704, the eNB1transmits a cell remove indication which indicates that the eNB1 hasremoved its cell. Accordingly, the eNB1 may inform the eNB2 the eventthat the eNB1 completed to remove its cell.

Referring to FIG. 13-(b), in step S710, the eNB1 decides to remove itscell. In step S711, the eNB2 decides a handover, and accordingly, instep S712, the eNB2 transmits the HANDOVER REQUEST message to the eNB1.That is, the eNB1 receives the HANDOVER REQUEST message from the eNB2right after the eNB1 decided to remove its cell. Therefore, in stepS713, the eNB1 transmits the HANDOVER PREPARATION FAILURE message to theeNB2 with the cause information to indicate that the target cell (e.g.,cell of the eNB1) is planned to be removed. In step S714, the eNB1transmits a cell remove indication which indicates that the eNB1 willremove its cell. Accordingly, the eNB1 may inform the eNB2 the plan toremove its cell after making cell-removing decision. In step S715, theeNB1 remove its cell.

According to embodiments described in FIG. 11 to FIG. 13, when an eNB isinitiating or executing its cell splitting/merging, upon receiving theHANDOVER REQUEST message targeting the cell(s) from its neighboring eNB,the eNB transmits the neighboring eNB the HANDOVER PREPARATION FAILUREmessage with the cause that informs the target cell is on the cellsplitting/merging initiation or execution. After receiving the HANDOVERPREPARATION FAILURE message, the neighboring eNB may wait for a certaintime as detecting the target cell's status and may transmit the HANDOVERREQUEST message to the eNB again. If the cell splitting/merging iscompleted, the eNB executes the conventional handover procedure, and ifnot the eNB transmits the neighboring eNB the HANDOVER PREPARATIONFAILURE message again.

Alternatively, when an eNB is initiating or executing its cellsplitting/merging, upon receiving the HANDOVER REQUEST message targetingthe cell(s) from its neighboring eNB, the eNB may transmit theneighboring eNB the HANDOVER REQUEST ACKNOWLEDGE message, which containsa timer value and the new cell identity (ID) to be assigned to thetarget cell after the splitting/merging completion. The neighboring eNBthat receives the HANDOVER REQUEST ACKNOWLEDGE message may wait for theduration of the timer value and then executes the rest of handoverprocedure, such as handover command to the UE, toward the target cellhaving the new cell ID.

FIG. 14 shows a wireless communication system to implement an embodimentof the present invention.

A first eNB 800 includes a processor 810, a memory 820, and a radiofrequency (RF) unit 830. The processor 810 may be configured toimplement proposed functions, procedures, and/or methods in thisdescription. Layers of the radio interface protocol may be implementedin the processor 810. The memory 820 is operatively coupled with theprocessor 810 and stores a variety of information to operate theprocessor 810. The RF unit 830 is operatively coupled with the processor810, and transmits and/or receives a radio signal.

A second eNB 900 includes a processor 910, a memory 920 and an RF unit930. The processor 910 may be configured to implement proposedfunctions, procedures and/or methods described in this description.Layers of the radio interface protocol may be implemented in theprocessor 910. The memory 920 is operatively coupled with the processor910 and stores a variety of information to operate the processor 910.The RF unit 930 is operatively coupled with the processor 910, andtransmits and/or receives a radio signal.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

The invention claimed is:
 1. A method for transmitting coveragemodification information in a wireless communication system, the methodcomprising: transmitting, by a first base station, the coveragemodification information which indicates that removing a cell managed bythe first base station is planned, to a second base station; and afterthe first base station transmits the coverage modification informationto the second base station, removing, by the first base station, thecell managed by the first base station.
 2. A first base station in awireless communication system, the first base station comprising: atransceiver configured to transmit or receive a radio signal; and aprocessor, coupled to the transceiver, that: controls the transceiver totransmit coverage modification information which indicates that removinga cell managed by the first base station is planned, to a second basestation, and after the coverage modification information is transmittedto the second base station, removes the cell managed by the first basestation.
 3. The method of claim 1, further comprising: determining toremove the cell managed by the first base station, in an active antennasystem (AAS); receiving a handover request message from the second basestation immediately after determining to remove the cell, before thefirst base station removes the cell; and transmitting a handoverpreparation failure message including cause information, to the secondbase station, in response to the receiving the handover request message,wherein the cause information indicates that removing the cell isplanned; and after the transmitting the handover preparation failuremessage, removing the cell managed by the first base station.
 4. Themethod of claim 1, wherein the cell managed by the first base station isremoved by using an active antenna system (AAS).